As is known in the art, a photovoltaic (PV) system is a system which uses one or more solar cells (also sometimes simply referred to as “cells”) to convert light into electricity. Due to the relatively low voltage of an individual solar cell (typically on the order of 0.5 volts (V)), several solar cells are often combined into PV panels (also sometimes referred to as PV modules) which are in turn connected together into an array.
The electricity generated can be either stored, used directly (so-called island/stand-alone plant) or fed into a large electricity grid powered by central generation plants (so-called grid-connected/grid-tied plant) or combined with one or many domestic electricity generators to feed into a small grid (a so-called “hybrid plant”).
As is also known, a PV system which is connected to an independent grid (e.g. the public electricity grid) and which is capable of feeding power into the grid is often referred to as a grid-connected system. This is a form of decentralized electricity generation. In the case of a building mounted grid connected PV system (e.g. a residential or office building), the electricity demand of the building is met by the PV system and any excess electricity is fed into the grid. The feeding of electricity into the grid requires the transformation of direct current (DC) into alternating current (AC). This function is performed by an inverter.
On the AC side, grid-connected inverters supplies electricity in sinusoidal form, synchronized to the grid frequency, limit feed in voltage to no higher than the grid voltage.
On the DC side, the power output of a module varies as a function of the voltage in a way that power generation can be optimized by varying the system voltage to find a so-called maximum power point. Most inverters therefore incorporate maximum power point tracking.
The AC output is typically coupled across an electricity meter into the public grid. The electricity meter preferably runs in both directions since at some points in time, the system may draw electricity from the grid and at other points in time, the system may supply electricity to the grid.
As is also known, an initial use of power from AC electricity distribution system was focused on automating machinery and to provide lighting. While then basic designs are still in use a century later (albeit more refined), such designs have been increasingly replaced by more advanced implementations, or alternatives that don't utilize AC signals natively. Instead, modern grid connected machines and devices convert the grid voltage provided thereto to a more appropriate form such as direct current (dc) voltage or high-frequency modulated AC voltage. Additionally, much electricity used today is in residential and commercial environments, a shift from the primarily industrial usage a century ago.
As is also known, grid tied inverters for photovoltaic (PV) systems have also evolved since their inception. Grid tied inverters for PV systems traditionally managed large series-parallel connected arrays and then evolved to also handle lower power strings of panels, and have further evolved to operate with a single PV module. Inverters which operate with a single PV module are referred to as micro-inverters or module integrated converters (MIC). Micro-inverters provide a number of benefits including ease of installation, system redundancy, and increased performance in partially shaded conditions.
One drawback of such systems, however, is the difficulty in obtaining the same efficiencies as inverters which manage multiple modules in series at higher power levels. For example, a single 72-cell panel having a nominal output voltage of 36 volts (V) requires a transformation ratio to interface with a root mean square (RMS) grid voltage of 240 V AC, which is much larger than that required by a series string of ten panels.
Connecting electronic devices to an AC distribution system is a well understood task, and significant work has been completed in both sourcing power from, and delivering power to the grid. Much of this work is focused on three-phase interconnection of varying line voltages, with power levels ranging from 10-500 kW, often for applications in motor drives, electric vehicle drive-trains, wind turbines, and uninterruptible power supply (UPS) systems. At these power levels, with a three phase distribution system, efficiencies up to 98% are achievable. This is in contrast to electrical systems found in commercial and residential environments, which often operate on single or split-phase systems at a significantly lower power level. This results in an increased difficulty to maintain high energy conversion efficiencies.
One challenge for such single phase converters is the DC plus sinusoidally varying power transfer to the grid. This results in the need for a converter capable of processing power from zero to double the average power, at twice the line frequency.